+P ammo is loaded to pressures that exceed the design specifications for the R-9. The R-9 is Rohrbaugh's attempt at making the smallest possible delivery system for standard pressure 9mm rounds. It was trial and error to be sure.
If you run it at pressures exceeding the design specs, bad things can and likely will happen. And when it does -- there is no warranty coverage.
Just don't do it -- ever. At a minimum you will batter the heck out of the gun absorbing the momentum of the slide. At worst.... who knows?
I am not sure 9mm +P will really do anything more for you out of the R-9's short barrel. If you really want more noise, recoil and flash -- get a snubby .357 Magnum.